The present invention relates to a method and apparatus for controlling lateral unstable movement and camber of a strip being rolled.
Under various rolling conditions, there occurs a phenomenon that a strip being rolled cannot remain at the center portions of work rolls and is forced to move toward one end of the work rolls as shown in FIG. 1. Such phenomenon is referred to as "the lateral unstable movement of the strip" in this specification.
Referring to FIG. 1, the lateral unstable movement of a strip a will be described in detail. FIG. 1 shows that the strip a is caused to move toward the right from the center portion of work rolls b by some rolling disturbance. Then the roll gap is not uniform. That is, the roll gap at the right becomes greater than that at the left. However, the velocity of the work roll b is the same at the right and left so that the volume of material (strip) entering the right gap between the work rolls b per unit time becomes greater than the volume of material (strip) entering the left gap between the work rolls b per unit time. When the strip a enters the work rolls b, the cross section of the strip a is uniform so that the right portion of the strip a is more quickly drawn into the work rolls b than the left portion of the strip a. As a result, as shown in FIG. 2, when the strip material is entering the work rolls b, it is forced to move toward the right (.DELTA.x) so that when the strip material leaves the work rolls b, a camber (.DELTA.y) is produced. As a consequence, the difference between the right and left roll gaps is further increased so that the strip a is quickly forced to move toward the right. As a result, the rapid lateral displacement of the strip occurs and the camber is increased.
In order to prevent the lateral unstable movement of the strip a and the resulting camber, it is effective to roll a strip such that the strip rolled has a positive crown. However, because of the recent demands for high quality and yield, the positive crown must be suppressed and the cross sections in both the lengthwise and widthwise directions must be as uniform as possible. When a strip is rolled so as to attain such results, the lateral unstable movement tends to occur very frequently and the stable rolling operation becomes very difficult.
In order to prevent this instability, there has been demonstrated a method in which the output signals from load cells c and d (See FIG. 1) disposed at the right and left sides of the mill e are used so as to detect whether or not the lateral movement of the strip a occurs so that, if the output from one of the load cells c and d is increased, the rolling force may be increased on the side where the rolling load is increased. However this method has a defect that the output from a load cell contains both a component due to the lateral unstable movement of the strip a and a component due to the increase or decrease in rolling force. Futhermore a control system used is unstable so that vibration tends to occur. In addition, the control of the lateral displacement of the rolled strip a is not accurate in practice.
Even when the lateral unstable movement of the strip can be prevented, a camber is produced because of non-symmetrical-thickness variation of incoming strip in the widthwise direction and an initial camber. A camber is produced further because of temperature difference across the width. However, there has not been proposed any satisfactory solution to these problems. The only countermeasure is that the strip is so rolled that it has a sufficient width and thereafter undesired portions of the rolled strip are cut off so that a rolled strip with a desired plane shape (i.e. shape seen from the above) is provided. As a result, the yield is considerably decreased.
The present invention is made to overcome the above and other problems encountered in the prior art rolling methods and has for its object to prevent the lateral unstable movement of a strip by a stable control method and to correct a camber, whereby the quality of the rolled strip can be improved, the yield can be increased and the rolling operation can be carried out in a stable manner.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.